LED light sources are attractive alternatives to traditional incandescent, fluorescent, or halogen lamps because of their high light output and low power consumption. LED lamps, however, require specialized driver and/or control circuits in order to properly supply power to the LEDs (typically, via a regulated current) using traditional power sources (e.g., an AC line voltage). As a further constraint, safety standards created by organizations such as UL and CE require that hazardous voltages (e.g., voltages above approximately 50 volts) must be isolated from users.
A popular LED driver circuit that fulfills these requirements uses a flyback converter, which applies the input voltage to a primary side of a flyback transformer (i.e., a charge-storing transformer) to induce a current therein. A switch periodically shuts off the application of the input voltage, during which time the flyback transformer discharges its stored charge as a current through its secondary side. This secondary-side current is used to drive the LEDs. As an added benefit, the separation between the primary and secondary sides of the flyback transformer provides the electrical isolation required by the safety standards.
Because the LED drive current requires regulation, however, the flyback-converter circuit must also sense any variations in the secondary-side current and adjust the power delivered to the primary side of the flyback transformer accordingly. For example, if the secondary-side current is too low, the primary-side control circuit may increase the amount of time per cycle that the switch is on and thereby apply more input power to the primary side. The sensed secondary-side LED current must be therefore transmitted back to the primary side, and it must be done while preserving the primary/secondary electrical isolation.
An existing circuit 100 for feeding back the sensed LED current to the primary side of the circuit is shown in FIG. 1. An input line voltage 102 is applied to a filter and rectifier 104 and thereafter to a primary side 106 of a flyback transformer 108. A primary-side control circuit 110 periodically shuts off a transistor switch 112, at which time the secondary side 114 applies a current to the LEDs 116 (through a rectifier diode 118 and a filter capacitor 120). A current-sense circuit 122 senses the current through the LEDs 116 and sends a corresponding sensing signal through an opto-isolator 118. The opto-isolator 118 passes the signal across the isolation barrier using a light-emitting diode and a photodiode, thereby preserving the electrical isolation. The primary-side control circuit 110 receives the feedback signal from the opto-isolator 118 and adjusts the switching time of the switch 112 accordingly.
There are a number of drawbacks to the use of the opto-isolator 118, however. The accuracy of the current regulation depends on the quality of the analog signal passed therethrough; if the opto-isolator 118 is poorly calibrated, out of specification, and/or changes or degrades over time, the light produced by the LEDs 116 may undesirably vary and/or the lifespan of the LEDs 116 may shorten. These issues may be mitigated (but not eliminated) by the use of a higher-quality opto-isolator 118, but the higher cost of such a component may be undesirable or prohibitive.
The shortcomings of the use of the opto-isolator 118 are exacerbated if the LEDs 116 are to be used with a dimming signal. Any errors introduced into the feedback signal by the opto-isolator 118 produce erratic, inconsistent, or time-varying levels of light output by the LEDs 116 for a given dimmer setting. Furthermore, if one dimmer signal is used to control multiple LED lamps, differences in each of the opto-isolators in each lamp may produce different levels of output light from each lamp.
A need therefore exists for an LED lamp driver circuit that complies with safety standards while providing accurate and consistent current regulation and dimming control.